Crude oil is a complex mixture of thousands of chemical species, most of which are hydrocarbons, i.e., they are made of carbon and hydrogen. Some chemical species in crude oil contain other elements (referred to as “hetero elements”), such as sulfur, nitrogen, or metals such as vanadium or nickel. Crude oils from different locations on the earth have vastly different composition; the types and relative amounts of compounds they contain can vary greatly. For example, “heavy” crude oils contain a relatively higher amount of large hydrocarbon molecules whereas “light” crude oil contains a relatively higher amount of small hydrocarbon molecules. “Sweet” crude oil contains little sulfur (typically less than about 0.5 weight percent) whereas “sour” crude oil contains a higher amount of sulfur (typically 1-2 weight percent or more).
Crude oil itself is generally not very useful; it must be refined and processed to yield valuable products. Refining and processing involves subjecting the crude oil to various separations and chemical reactions that ultimately yield a spectrum of useful products such as transportation fuels (aviation gasoline, automobile gasoline, and diesel fuel), heating oil, and kerosene, asphalt and petroleum coke.
The highest value products, such as transportation fuels, are typically obtained in the highest quantities and with the least effort from light sweet crude oils. However, light sweet crude oil sells at a premium because of its desirability. The high price of light sweet crude oil has incentivized efforts to obtain valuable petroleum products from lower grades of crude oil, i.e., heavier crude oil having more sulfur and metals. Such crude oils require more complex refining and processing.
Petroleum coke (also referred to as petcoke and referred to herein simply as coke) is a high carbon product of petroleum refining. Coke is obtained by heating the heavy residue left over from distillation processes (also referred to as resid) in the presence of steam to produce a solid, carbonaceous material. The marketability of coke depends on the amount of sulfur it contains. Coke with a relatively high sulfur content is referred to as fuel grade coke and is sold as fuel for coal-burning boilers, typically for power generation. Coke with a relatively low sulfur content can be used to make electrodes that are valuable for use in dry cells and in industrial electrical process, such as the production of aluminum.
Anode grade coke generally must have a sulfur content less than 3 weight percent, a nickel content less than 200 ppm, a vanadium content less than 350 ppm and a total metals content less than 500 ppm. In addition to heteroatom content, anode grade coke is subject to other constraints. For example, anode grade coke that is suitable for making carbon anodes suitable for aluminum manufacture preferably has an HGI grindability index greater than 70, a bulk density of at least 50 lbs/ft3, and a volatile carbonaceous material content of less than 10 or 12 weight percent. It is more desirable to produce anode grade coke since this is a higher value product than fuel grade coke. Anode grade coke is about three to five times more valuable than fuel coke.
The quality of crude oil that a refining process uses as a feedstock largely determines the grade of coke obtained from the process. Light sweet crude produces a higher yield of anode grade coke. However, as economic forces drive refiners to using heavier and more sour crude, anode grade coke is more difficult to obtain. The distillation of the crude tends to concentrate sulfur and other contaminants into the resid.
Hydroprocessing generally refers to hydrotreating and hydrocracking processes. Hydroprocessing is a method of improving the quality of distillate products and occasionally treating residues from distillation processes. Hydroprocessing involves reacting the distillate products and at times residue from distillation with hydrogen in the presence of a catalyst to remove sulfur. Hydrocracking is used to convert the high boiling distillates and at times petroleum residue into a higher proportion of more valuable lower-boiling products. The residue remaining after the lower-boiling products are removed from the hydroprocessing unit effluent generally has a lower sulfur and metal content.
It has been suggested to hydrotreat the distillate residues obtained from high sulfur crude to remove the sulfur and provide a higher grade of coke. Unfortunately, hydrotreating of the petroleum residue affects the physical characteristics of the resulting resid, rendering it unsuitable for the anode manufacturing process. Therefore, for the production of anode grade coke, feedstocks have been historically limited to virgin residues with inherently low sulfur and metals content.
There is a need, therefore, for improved methods of processing lower grade crude to obtain higher amounts of valuable products, such as anode-grade coke.